Optical fiber resistant to hydrogen-induced attenuation

ABSTRACT

Improved single-mode optical waveguide fibers having a central core region, surrounded by an inner cladding region through which light at a chosen signal wavelength will propagate to an appreciable degree along with propagation of same in the central core region, the inner core region further surrounded by an outer cladding region, the improvement comprising germanium dioxide in the inner cladding region at a concentration within the range of about 0.005 percent by weight to about 1 percent by weight of said inner cladding region, effective to significantly reduce the concentration of oxygen atoms in the inner cladding region which are available to form defects that cause hydrogen-induced attenuation. Also provided are core preforms, overclad preforms, and processes for making the fibers, core preforms and overclad preforms.

This is a division of application Ser. No. 08/728,713, filed Oct. 11,1996 now U.S. Pat. No. 5,838,866, which claims priorty of U.S.Provisional Application Ser. No. 60/006,217 filed on Nov. 3, 1995.

BACKGROUND

This invention relates in general to optical fibers, and, in particular,to an optical fiber that resists attenuation caused by hydrogen andmethods of making it.

The low attenuation and dispersion characteristics of optical fibers areadvantageously employed to form long repeaterless links, although thereis a certain amount of attenuation present in any fiber. Suchattenuation ultimately requires reamplification of the light carried bythe fiber. In certain circumstances it is desired to use a largepercentage of the loss budget made available by the low loss(attenuation) of fiber by using long repeaterless fiber links, therebyproviding very little safety factor. If after the fiber is placed inservice, attenuation in the fiber significantly increases at thetransmitting wavelength system operation can be interrupted.

Studies have found that attenuation of installed fibers is caused, inpart, by hydrogen entering the fiber, especially the core. There areseveral known hydrogen induced attenuation effects: (1) interstitialhydrogen, which is directly proportional to the partial pressure ofhydrogen in the ambient atmosphere, and is reversible; (2) increases inphosphorus hydroxyl absorption (1300-2000 nm) which precludes the use ofP₂ O₅ as a dopant, except in low (less than 0.1 wt. %) concentration;(3) under high temperature-long term H₂ exposure, there results a highoptical absorption at short wavelengths that has an extensive tailextending through the visible and into the infrared region; (4)transient absorption that occurs when H₂ first arrives in the fiber coreregion with most notably peaks at 1330, 1440, and 1530 nm; and permanentabsorption that occurs due to Si--O--O--H--H at 1380 nm.

Others have made attempts to mitigate the hydrogen attenuation problem.See, for example, Blankenship U.S. Pat. No. 5,059,229, assigned toCorning Incorporated which describes a process for post-treating a fiberby exposure to hydrogen to reach a stable, albeit elevated attenuationlevel; and demonstrating no further increased attenuation when the fiberis subsequently exposed to a hydrogen containing atmosphere after beingplaced in service. Despite this symptomatic treatment and other efforts,the problem of hydrogen-induced attenuation persists.

One principal cause of light attenuation in optical fibers is hydroxylgroups, which produce a very strong optical absorption peak near 1380nm. Much work has been done as evidenced by the published literature inan effort to reduce the presence of such species. It is conventionallyknown, for example, to dry a porous glass soot preform duringconsolidation in the presence of chlorine, which reacts with waterpresent in the glass to form hydrogen chloride gas which is thensimultaneously removed from the preform at high temperatures, thusreducing the concentration of hydroxyl ions in the glass.

Even granting such measures, other sources of attenuation persist. Information of silica glasses (particularly during consolidation of thecore preform and during fiber draw), peroxyl linkages (--Si--O--O--Si--)may occur, because excess oxygen becomes trapped within the glass. Theseperoxyl linkages can decompose, yielding reactive --Si--O--O-- sites. Ifhydrogen subsequently enters the glass, it can react with the--Si--O--O--species, forming Si--O--O--H--H species which absorb at 1530nm and could therefore adversely affect operation at 1550 nm. TheSi--O--O--H--H species subsequently lose a hydrogen atom and formSi--O--O--H₂ which absorbs at 1380 nm. Additionally, Si--Si defects mayoccur. These can decompose to Si--Si-- radicals, and excess oxygen canreact with them to form Si--O--O radicals. We also suspect thatgermanium may incorporate itself into the Si--Si defects.

We have now found that germanium dioxide can control theattenuation-increasing effects of hydrogen migration into thelight-carrying regions of an optical fiber, by scavenging excess oxygenwhich would otherwise form reactive species, thereby preventing thereaction of such oxygen with migrating hydrogen to form hydroxyl groups.The germanium is introduced into the soot deposition flame in reactiveform, e.g., germanium tetrachloride. Upon burning of the reactantsincluding germanium tetrachloride to produce glass soot during preformlay down the germanium tetrachloride will react with oxygen to formgermanium dioxide. Germanium dioxide deposited by the flame depositionprocess is not a stoichiometric compound because it contains fewer than2 oxygen atoms for each germanium atom. Hence, the germanium "dioxide"can scavenge excess oxygen from the preform glass during consolidationand fiber drawing.

It is known conventionally to use germanium dioxide as a dopant in thecore glass of an optical fiber preform, for the purpose of increasingthe core refractive index--thereby facilitating the transmission oflight through the ultimate optical fiber. During the process ofconsolidating the porous glass soot for the core, the chlorine used fordrying has the side effect of reacting with germanium dioxide to producegermanium tetrachloride. Thus mobilized, the germanium in tetrachlorideform can migrate outward from the core and redeposit as germaniumdioxide.

In a small-sized preform, the migration of germanium tetrachlorideoutward throughout the light-carrying regions of the fiber preform dueto reaction with the chlorine may be sufficient to provide enoughgermanium dioxide to control the excess oxygen which would otherwise beavailable for reaction with later-migrating hydrogen. However, thisbeneficial effect of the chlorine drying step depends on the diameter ofthe preform being dried and consolidated. The greater the preformdiameter, the less effective the chlorine drying step will be in actingon available germanium dioxide in the core to spread it out into outerlight carrying regions. As efficiencies of scale are achieved withdrawing optical fiber of ever-increasing volume and hence increasingdiameter, the need to directly address scavenging of excess oxygenthrough the light-carrying regions of the ultimate optical fiberaccordingly becomes critical. We have found that in preforms having adiameter in excess of 105 millimeters (mm), the chlorine-inducedmigration of core-deposited germanium dioxide can be insufficient.

SUMMARY OF INVENTION

The conventional art has consistently taught against adding germaniumdioxide other than to the core glass of an optical fiber preform. Inmultimode fiber, light travels solely in the core. It does so becausethe index of refraction of the core glass is designed to be higher thanthat of the cladding. Adding germanium dioxide to the cladding wouldinevitably raise the refractive index of the cladding, potentiallydestroying the light-carrying potential of the optical fiber.Single-mode optical fiber operates somewhat differently. By single-modeoptical we mean an optical fiber as conventionally so designated, i.e.,an optical fiber which propagates only the two mutually orthogonal modesof the HE11 mode of light, at a chosen signal wavelength. Again it iscritical that the refractive index of the core be higher than that ofthe cladding. However, in this case the light travels in both the coreand the cladding. The conventional teachings still point to avoiding thepresence of germanium dioxide in the cladding, because that will tend toincrease the refractive index of the cladding and ultimately destroy thelight-carrying potential of the optical fiber.

However, we have now found that germanium dioxide can be added to theportion of the cladding of a single-mode optical fiber that is intendedto carry light in small concentrations, which will act to scavengeotherwise labile excess oxygen in the glass to control theattenuation-increasing impact of later-migrating hydrogen on the opticalfiber, while raising the refractive index of the glass to a degreeinsufficient to result in significant adverse impact on the single-modelight-carrying potential of the fiber.

More particularly, this invention provides a single-mode optical fibercomprising a central core, an inner cladding surrounding the core, andan outer cladding surrounding the inner cladding. The core is dopedconventionally to raise its refractive index. The inner cladding andouter cladding may be made from essentially the same material, exceptthat the inner cladding is doped with a small concentration of germaniumdioxide. In preferred embodiments, the core dopant also comprisesgermanium dioxide. The concentration of germanium dioxide in the innercladding broadly ranges from about 0.005 percent by weight to about 1percent by weight; preferably about 0.1 to about 0.5 percent; and mostpreferably about 0.1 to about 0.3 percent. The invention also providesprocesses for making core and overclad preforms useful in making suchoptical fibers, and such core and overclad preforms.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an optical fiber made accordancewith the invention.

FIG. 2 is a graph showing the percentage concentration of germaniumdioxide in the annular inner cladding of the optical fiber of FIG. 1.

FIGS. 3 and 4 illustrate the application of the core and inner claddingregions of glass soot to a mandrel;

FIG. 5 is a cross-sectional view of a dense glass blank that is formedon the mandrel of FIGS. 3, 4;.

DETAILED DESCRIPTION

In accordance with a known technique for forming single-mode opticalfibers, a core cane comprising a fiber core region and an inner claddingregion is initially formed. The core cane is overclad with additionalcladding material to form a preform that is drawn into an optical fiber.As shown in FIG. 1, the optical fiber has a central core 10 that isdefined by an outer surface 11. Inner cladding region 22, which forms anannulus around core 10, has an inner surface 13 formed on the outersurface 11 of core 10. Inner cladding region 22 also has an outersurface 15. Outer cladding region 17 surrounds region 22. In onecommercial embodiment of a single-mode fiber of the above-describedtype, the diameter of the core 10 is approximately 8.8 microns, theradial thickness of the inner cladding region 22 is approximately 6.1microns and the radial thickness of the outer cladding region 17 isapproximately 52 microns.

The material of the inner cladding region 22 is normally pure silica. Itis also known to add dopants to inner cladding region 22 that lower theindex of refraction of the inner cladding region 22. By reducing theindex of refraction of the inner cladding region 22, the differencebetween the indexes of refraction of the core 10 and the inner claddingregion 22 are substantially increased. So, following conventionalteachings, it is normally undesirable to raise the index of refractionof the inner cladding region 22.

Nevertheless, adding relatively small amounts of germanium dioxide tothe inner cladding region 22 significantly reduces the later incidenceof hydrogen-induced attenuation. The results of one experiment are shownin FIG. 2 (OD=Outer Diameter). There are shown the relative preparedamounts of germanium dioxide in the inner cladding annulus for a preform(including outer cladding) having an unconsolidated diameter of 90millimeters without germanium dioxide doping in the inner claddingregion, and a similar preform having an unconsolidated diameter of 125millimeters with and without germanium dioxide doping in the innercladding region. As expected, some of the germanium dioxide from thecore diffused into the inner cladding region 22 in a region next to theinterface 11-13 of the core 10 and the inner cladding region 22.However, the germanium dioxide concentration in the 90 mm blank and inthe 125 mm blank decreased almost to zero at a distance of about 30% ofthe radius of the inner cladding region annulus 22 from the innersurface 13. The 125 mm blank with germanium dioxide doping in the innercladding region maintained a level of germanium dioxide concentration atapproximately 0.5% by weight in an annular region extending to the outersurface 15 of the inner cladding region. The portion of the innercladding region near the core had higher concentrations of germaniumdioxide, due to migration of germanium in tetrachloride form from thecore to the inner cladding region during drying and consolidation.

The comparative results of FIG. 2 are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                    Mean Hydrogen                                                                 Induced                                                                       Attenuation -                                                                 [dB/km max @ Number   Number                                                  1530 nm](STD of Core  of Optical                                  Process     Deviation of Test                                                                          Preforms Fibers                                      Description Results)     Tested   Tested                                      ______________________________________                                        90 mm OD - No                                                                             0.005 (0.003)                                                                              30       30                                          GEO.sub.2 deliberately                                                        doped In Inner                                                                Clad                                                                          125 mm OD - No                                                                            0.573 (0.081)                                                                              7        16                                          GEO.sub.2 deliberately                                                        doped In Inner                                                                Clad                                                                          125 mm OD + Ge                                                                            0.030 (0.036)                                                                              7        19                                          doping - In Inner                                                             Clad                                                                          ______________________________________                                    

Further experiments have shown that the concentration of germaniumdioxide in the inner cladding region of an optical fiber preform shouldbe at least about 0.005 percent by weight; and that concentrations aboveabout 1 percent by weight will lead to inordinate increases in therefractive index of the inner cladding region. Preferably, the germaniumdioxide concentration in the inner cladding region ranges from about 0.1to about 0.5 percent; and most preferably about 0.1 to about 0.3percent. We define the boundary of the inner cladding region as beingthe outer limit of the portion of the ultimate optical fiber throughwhich an appreciable portion of light directed through the end of thefiber is transmitted (thereby making reduction of hydrogen-inducedattenuation of significance there).

These relatively small concentrations of germanium dioxide used in theinner cladding region do not have an undue effect upon the transmissionof light within the core and inner cladding region.

We now describe preferred embodiments of processes for making core andfiber preforms as well as optical fibers. It is to be noted that thedrawings are illustrative and symbolic of the invention, and there is nointention to indicate scale or relative proportions of the elementsshown therein. Further, it is to be noted that the present inventionexpressly contemplates single-mode waveguides since the problem ofhydrogen-induced attenuation does not occur in multimedia waveguides.The present invention also contemplates optical waveguides having coreswith either a constant gradient or otherwise varied index of refraction.

Optical waveguide soot preforms may be conventionally prepared e.g., inaccordance with the methods illustrated in FIGS. 3 and 4. A coating 10of glass soot is supplied to cylindrical mandrel 12 by means of flamehydrolysis burner 14. Fuel gas and oxygen or air are supplied to burner14 from a source (not shown). This mixture is burned together withliquid precursors to the glass soot, such as silicon tetrachloride or apolyalkylsiloxane (e.g., octamethyl-cyclotetrasiloxane) to produce flame16 which is emitted from the burner. A gas-vapor mixture of fuel gas,oxygen and soot precursors is oxidized with flame 16 to form a glasssoot that leaves the flame in a stream 18, which is directed towardmandrel 12. The first soot of coating (many layers) deposited on mandrel12 forms the core 10 of the optical fiber. The flame hydrolysis methodof forming soot coatings on cylindrical mandrels is described in greaterdetail in U.S. Pat. Nos. Re 28,029 and 3,823,995. Mandrel 12 issupported by means of handle 20 and may be rotated and translated asindicated by arrows adjacent thereto in FIG. 3 for uniform deposition ofsoot.

A second coating of soot (many layers) is applied over the outsideperipheral surface of first coating 10 as shown in FIG. 4. The secondcoating will form the inner cladding region 22. In accordance withwell-known practice the refractive index of inner cladding region 22 ismade lower than that of coating 10 (core region) by changing thecomposition of the soot 24 being produced in flame 16. This can beaccomplished by changing the concentration or type of dopant materialbeing introduced into the flame, or by omitting a dopant material.Mandrel 12 may again be rotated and translated to provide a uniformdeposition of inner cladding region 22, the composite structureincluding first coating 10 (core region) and second cladding 22constituting an optical waveguide soot core preform 41.

According to the invention, the process for application of the secondcoating of soot ultimately forming the inner cladding region 22, ismodified from conventional teachings by the introduction of suitableconcentrations of a germanium precursor (such as germaniumtetrachloride) to yield the prescribed concentrations of germaniumdioxide in the inner cladding region of the preform and the ultimateoptical fiber. In preferred embodiments, the concentration of thegermanium precursor in the soot precursor composition ranges from about0.003 to about 0.6 mole percent, more preferably about 0.03 to about 0.3mole percent, and most preferably, about 0.06 to about 0.2 mole percent.In another preferred embodiment, the composition of the inner claddingsoot precursor composition is maintained constant during the depositionof the inner cladding region glass soot. We note here that although theabove description has been provided to illustrate the process aside fromthe germanium addition to the inner cladding region is entirelyconventional. Hence, modifications to the conventional process steps asknown to those of ordinary skill in the art can be employed. Forexample, any of the various lay down processes can be used, includingbut not limited to outside vapor deposition, inside vapor deposition,vapor axial deposition, modified chemical vapor deposition, or plasmaoutside inside deposition.

In the manufacture of optical fibers, the materials of the core andcladding (inner and outer) regions of the optical fiber should beproduced from a glass having minimum light attenuation characteristics,and although any optical quality glass may be used, fused silica is aparticularly suitable glass. For structural and other practicalconsiderations, it is desirable for the core and cladding glasses tohave similar physical characteristics. Since the core glass must havehigher index of refraction than the cladding for proper operation, thecore glass may desirably be formed of the same type of glass used forthe cladding and doped with a small amount of some other material toslightly increase the refractive index thereof. For example, if purefused silica is used as the cladding glass, the core glass can consistof fused silica doped with a material to increase its refractive index.Precursors to silica can include, by way of examples: silicontetrachloride, polyalkylsiloxanes such as hexamethylcyclotrisiloxane,and polyalkyicyclosiloxanes such as octamethylcyclotetrasiloxane,hexamethylcyclotrisiloxane and decamethylcyclopentasiloxane.

Many suitable materials have been used as dopants alone or incombination with each other to increase the refractive index of fusedsilica. These include, but are not limited to, titanium oxide, tantalumoxide, aluminum oxide, lanthanum oxide, phosphorus oxide and germaniumdioxide. A core of germanium dioxide-doped fused silica isadvantageously provided with a cladding layer of fused silica.Precursors to germanium dioxide can include germanium tetrachloride.

Removing the mandrel 12 results in a hollow, cylindrical porous sootcore preform 41, such as that illustrated in FIG. 5. Preform 41comprises first and second porous soot glass layers 10 and 22,respectively, the refractive index of layer 10 being greater than thatof layer 22. It is also possible although less preferred, to draw corecane after depositing and consolidating core layer 10 only.

The core preform 41 is then consolidated, deposited with an overcladdingto form the outer cladding region and consolidated again. Theconsolidation and overcladding steps are well known in the art but arebriefly described for the sake of continuity. The preform 41 is a porousstructure with a texture that resembles chalk. It is consolidated byheating it in a furnace in a controlled manner generally in the presenceof chlorine and optionally helium to remove the pores. The chlorine isused to dry the preform; this may be performed prior to theconsolidation step (preferred) or simultaneously. The consolidated corepreform is then drawn to remove the center hole and produce core cane,and cut into suitable lengths for overcladding. Next the overcladding isapplied by depositing soot on a suitable length of core cane. Theovercladded preform is then consolidated to remove pores in theovercladding. Finally, the consolidated overcladded preform 81 is thendrawn into an optical waveguide fiber.

Conventional optical waveguide fiber technology which, will be readilyemployed by those of ordinary skill in the art in practicing theinvention, all of which is hereby incorporated herein by reference,includes by way of non-limiting examples the following.

As to raw materials useful as soot precursors, see: Dobbins U.S. Pat.No. 5,043,002; and Blackwell U.S. Pat. No. 5,152,819.

As to processes for the vaporization or nebulization of soot precursors,see: Antos U.S. Pat. No. 5,078,092; Cain U.S. Pat. No. 5,356,451;Blankenship U.S. Pat. No. 4,230,744; Blankenship U.S. Pat. No.4,314,837; and Blankenship U.S. Pat. No. 4,173,305.

As to burning soot precursors and laydown of core and cladding, see:Abbott U.S. Pat. No. 5,116,400; Abbott U.S. Pat. No. 5,211,732; BerkeyU.S. Pat. No. 4,486,212; Powers U.S. Pat. No. 4,568,370; Powers U.S.Pat. No. 4,639,079; Berkey U.S. Pat. No. 4,684,384; Powers U.S. Pat. No.4,714,488; Powers U.S. Pat. No. 4,726,827; Schultz U.S. Pat. No.4,230,472; and Sarkar U.S. Pat. No. 4,233,045.

As to the steps of core preform consolidation, core cane drawing, andoverclad preform consolidation, see: Lane U.S. Pat. No. 4,906,267; LaneU.S. Pat. No. 4,906,268; Lane U.S. Pat. No. 4,950,319; Blankenship U.S.Pat. No. 4,251,251; Schultz U.S. Pat. No. 4,263,031; Bailey U.S. Pat.No. 4,286,978; Powers U.S. Pat. No. 4,125,388; Powers U.S. Pat. No.4,165,223; and Abbott U.S. Pat. No. 5,396,323.

As to fiber drawing from a consolidated overclad preform, see: HarveyU.S. Pat. No. 5,284,499; Koening U.S. Pat. No. 5,314,517; Amos U.S. Pat.No. 5,366,527; Brown U.S. Pat. No. 4,500,043, Darcangelo U.S. Pat. No.4,514,205; Kar U.S. Pat. No. 4,531,959; Lane U.S. Pat. No. 4,741,748;Deneka U.S. Pat. No. 4,792,347; OhIs U.S. Pat. No. 4,246,299; ClaypooleU.S. Pat. No. 4,264,649; and Brundage U.S. Pat. No. 5,410,567.

We claim:
 1. In a process for fabricating a core preform having utilityin making single-mode optical waveguide fibers, comprising reacting acore soot precursor composition and depositing central core region glasssoot on a substrate, and then reacting an inner cladding soot precursorcomposition and depositing inner cladding region glass soot over saidcentral core region glass soot to yield said core preform, theimprovement comprising including in said inner cladding soot precursorcomposition, a precursor to germanium dioxide at a concentration withinthe range of about 0.03 to about 0.3 mole percent, effective to yieldinner cladding region glass soot comprising between about 0.1 percent byweight to 0.4 percent by weight of germanium dioxide beyond a 0.1normalized radial distance from an outer edge of a core of the opticalfiber.
 2. A process of claim 1, in which the composition of the innercladding soot precursor composition is maintained constant during thedeposition of inner cladding region glass soot.
 3. A process accordingto claim 1, in which said inner cladding soot precursor compositioncomprises a precursor to germanium dioxide at a concentration within therange of about 0.06 to 0.2 mole percent, effective to yield innercladding region glass soot comprising between about 0.1 percent byweight to about 0.3 percent by weight of germanium dioxide.
 4. A processaccording to claim 1, in which said inner cladding soot precursorcomposition essentially comprises a precursor to silicon dioxide and aprecursor to germanium dioxide.
 5. A process according to claim 1, inwhich the precursor to germanium dioxide is germanium tetrachloride. 6.A process according to claim 4, in which the precursor to germaniumdioxide is germanium tetrachloride.
 7. A process according to claim 4,in which the precursor to silicon dioxide is selected from the groupconsisting of silicon tetrachloride, hexamethylcyclotrisiloxane,hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, anddecamethylcyclopentasiloxane.
 8. A process according to claim 1, inwhich said core soot precursor composition comprises a precursor togermanium dioxide.
 9. A process according to claim 1, in which theconcentration of said precursor to germanium dioxide in the innercladding soot precursor composition is maintained essentially constantthroughout the step of depositing inner cladding region glass soot oversaid central core region glass soot.
 10. In a process for fabricating anoverclad preform having utility in making single-mode optical waveguidefibers, comprising (a) reacting a core soot precursor composition anddepositing central core region glass soot on a substrate, then (b)reacting an inner cladding soot precursor composition and depositinginner cladding region glass soot over said central core region glasssoot to yield a core preform, then (c) drying the core preform in thepresence of chlorine and consolidating it at high temperature, then (d)heating said core preform and drawing core cane therefrom, then (e)reacting an outer cladding soot precursor composition and depositingouter cladding region glass soot over a length of said core cane toyield said overclad preform, the improvement comprising including insaid inner cladding soot precursor composition, a precursor to germaniumdioxide at a concentration within the range of about 0.03 to about 0.3mole percent, effective to yield inner cladding region glass sootcomprising between about 0.1 percent by weight to 0.4 percent by weightof germanium dioxide beyond a 0.1 normalized radial distance from anouter edge of a core of the optical fiber.
 11. In a process forfabricating a single-mode optical waveguide fiber, comprising (a)reacting a core soot precursor composition and depositing central coreregion glass soot on a substrate, then (b) reacting an inner claddingsoot precursor composition and depositing inner cladding region glasssoot over said central care region glass soot to yield a core preform,then (c) drying the core preform in the presence of chlorine andconsolidating it at high temperature, then (d) heating said core preformand drawing core cane therefrom, then (e) reacting an outer claddingsoot precursor composition and depositing outer cladding region glasssoot over a length of said core cane to yield said overclad preform,then (f) consolidating the overclad preform in the presence of chlorine,then (g) heating said overclad preform and drawing waveguide fibertherefrom, the improvement comprising including in said inner claddingsoot precursor composition, a precursor to germanium dioxide at aconcentration within the range of about 0.03 to 0.3 mole percent,effective to yield inner cladding region glass soot comprising betweenabout 0.1 percent by weight to 0.5 percent by weight of germaniumdioxide beyond a 0.1 normalized radial distance from an outer edge of acore of the optical fiber.
 12. A process according to claim 1 whereinsaid normalized distance radial distance is beyond 0.2.
 13. A processaccording to claim 1 wherein said normalized radial distance is beyond0.25.